This invention relates to an impedance transformation circuit for transforming a two-pole electrical impedance, comprising first means for measuring the current flowing through the impedance during operation, second means coupled to the first means for providing a control signal corresponding to the measured current, and third means coupled to the second means for providing a further current proportional to the measured current to an input terminal of the circuit as a function of the control signal. Said further current therefore affects the impedance behaviour of the circuit at the input connection point.
An impedance transformation circuit of this type is used, inter alia, in voltage-controlled oscillators, adjustable electric filters, accurate low-frequency filters, frequency-dependent control circuits and the like, and is disclosed by U.S. Pat. No. 4,339,729.
In the impedance transformation circuit known in practice, the problem arises that the electrical behaviour of the first means for measuring the current through the impedance to be transformed is determined by other physical parameters or a different number of physical parameters from the electrical behaviour of the third means for providing the proportional further current at the input terminal. The accuracy of the impedance transformation is consequently dependent on spreads in the components used for the first and third means, differences in the temperature behaviour of said components, aging and the like. Although the parameter sensitivity can be reduced by using compensation techniques known per se, this leads quite quickly to relatively complex and expensive electronic circuits.